182 LIFE BY THE SEASHORE. 



antennules from any adhering particles of mud a very 

 necessary matter in animals living ' in muddy situations. 

 Both species have also a habit of holding the fringes out 

 at arm's length, and then sweeping them inwards ; it is 

 probable that in doing this food -particles are entangled 

 among the hairs, which thus serve as fishing-nets. 



There is no difficulty in distinguishing between the two 

 species of porcelain - crabs, for they are very unlike one 

 another in appearance. 



In P. platycheles the carapace is usually about half an 

 inch broad, and the length of the great claws is somewhat 

 over an inch. The upper surface in life is so densely 

 covered with fine mud that no colour is visible ; but the 

 under surface is whitish, and when carefully cleaned the 

 upper has a reddish tint. The hairy porcelain-crab, as it is 

 called, is a very interesting species on account of its adapta- 

 tions to a life in turbid water. It has been proved by 

 experiment that, hardy as the common shore crab is, water 

 containing mud is extraordinarily fatal to it. This is due to 

 the fact that the gills, as in all Crustacea, are external 

 structures, though they lie within a protecting gill-chamber. 

 In consequence they are exposed to the action of the mud 

 in the water of respiration. The particles settle on their 

 surface, and produce an effect which is, in a rough way, 

 analogous to the effect produced by deposits of dust in our 

 lungs, and this speedily asphyxiates the crab. If, therefore, 

 a crab is to live in sand or mud, it must have a special 

 mechanism to prevent the particles gaining access to the 

 gills. This is generally effected by the development of 

 hairs, placed on the general surface of the body, but 

 especially on the path of the respiratory current. The chief 

 point of entrance of the water to the gill-chamber is in most 

 crabs at the base of the great claws. If' you examine For- 

 cellana platycheles when at rest on a stone, you will see that 

 the legs in general, but the great claws in particular, are 

 densely fringed with hairs. These hairs, as is easily seen, 

 act as sieves, entangling the fine particles, and allowing pure 

 water only to pass through them. The sifting action of the 

 hairs is greatly increased by the fact that they are branched 

 and serrated, a point easily demonstrated by microscopic 

 examination. The third segment of the rudimentary legs is 



